The present invention relates to a method of carburizing metal workpieces, in particular, of steel.
The use of carbon enriching atmospheres for carburizing steel at temperatures from 1050.degree. C. to 800.degree. C. is known to increase the amount of carbon in a case of a certain depth from the surface which increases the hardness and the wear resistance of the case.
The atmospheres used generally contain about 20% CO, 40% H.sub.2 and 40% N.sub.2 and very small amounts of CO.sub.2 and water vapor. Such atmospheres are produced by generators known as endo gas generators or synthetically from gas or gasohol mixtures. The most common of these mixtures is methanol-nitrogen. Indeed, at the treatment temperatures used the methanol decomposes according to the reaction CH.sub.3 OH.fwdarw.CO+2H.sub.2 and a gaseous mixture having the above composition may be obtained.
The carburizing method is carried out in the following manner: the carbon monoxide present in the atmosphere reacts according the reaction: 2CO.revreaction.CO.sub.2 +C (1) and there is then a transfer of carbon atoms to the metal. The hydrogen present in the atmosphere also takes part in the carburization from the point of view of the speed of the process for it reacts with the carbon monoxide according to the reaction: CO+H.sub.2 .revreaction.C+H.sub.2 O (2).
Some carburizing treatments used up to the present, particularly those carried out in furnaces having a plurality of zones, comprise two successive phases: a first phase called the carburizing phase followed by a second phase called the diffusion phase. More specifically, such treatments comprise subjecting the workpiece to be treated in the carburizing zone to a temperature from 900.degree. C. to 940.degree. C. in a carbon enriched atmosphere having a carbon potential from 0.9% to 1.2% by weight for a certain period, then putting the workpiece into the diffusion zone where the process is allowed to proceed, the temperature decreasing gradually to between 880.degree. C. and 800.degree. C. and the carbon potential of the atmosphere decreasing to a value from 0.7 to 0.9% by weight. The workpiece is then quenched in a gaseous or liquid phase, e.g., an oil bath. The drop in temperature during the diffusion phase minimizes the problems of deformation during quenching.
The difficulties with such treatments are such that at the beginning of the process a high carbon potential is provided to increase the quantity of carbon without the carbon potential exceeding a limit beyond which soot is deposited on the workpiece. The limit, from 1% to 1.6% is a function of the temperature used. Also, at the end of the treatment it is advisable to have a lower carbon potential so as to diminish the amount of carbon on the surface, if not, during subsequent quenching the metallurgical properties of the workpiece are not satisfactory because there is then a residual austenitic phase present and therefore low surface hardness of workpiece. As has been seen, during carburizing treatments is posed the problem of providing and monitoring a predetermined carbon potential during each of the two phases of treatment.
According to the methods used up to now, to obtain the desired carbon potential during each of the phases in addition the mixture for forming CO, H.sub.2 and N.sub.2 constituents a hydrocarbon such as methane, propane or butane is injected in each of the zones and the flow rate of the hydrocarbon is regulated as a function of the amount of CO.sub.2 in the atmosphere. Indeed, given the reaction by which the workpiece being treated consumes CO (see reaction (1)) and the air intake into the treatment chamber, the CO.sub.2 concentration of the atmosphere has a tendency to increase and therefore the carbon potential tends to decrease. This is why the amount of CO.sub.2 in the atmosphere is monitored and the injection flow rate of the hydrocarbon is regulated as a function of the sought carbon potential. This regulation may also be effected by monitoring the amount of H.sub.2 O or O.sub.2 in the atmosphere.
Since the advent of synthetic atmospheres namely of nitrogen and methanol, an increase in the amounts of CO and H.sub.2 in the treatment atmospheres has been sought. In this respect particular reference should be made to the process disclosed in U.S. Pat. No. 4,306,918. This process comprises a first phase in which pure methanol is injected into the treatment furnace and the workpieces are maintained in an atmosphere having a carbon potential from 0.8% to 1.1%, then in a second phase injecting nitrogen (which is generally less expensive than methanol) into the furnace so the treatment is less expensive and maintaining the workpieces in an atmosphere having a carbon potential from 0.7% to 0.9%. This process provides great carburized case depths rather rapidly owing to the increase of the fuel constituents such as CO and H.sub.2 during the first phase. It is observed that according to U.S. Pat. No. 4,306,918 the ranges of carbon potential of the atmospheres used are conventional ranges, i.e., 0.7% to 1.1% an in all cases less than 1.1%, and the difference of the carbon potential between the two phases is small (0.2%).